Mobile communication systems are continuously growing denser in metropolitan areas. With an increasing number of subscribers requiring improved coverage and higher bandwidth, the area covered by a single base station shrinks from city districts to blocks and further down to streets and even parts of streets.
The backhaul networks used to aggregate and transport user traffic into the core network must evolve accordingly to match these new requirements. Denser and more highly occupied networks imply, among other things, that fixed point-to-point radio links, such as the widely deployed microwave radio links, must continue to grow more spectrum efficient and affordable in order to stay attractive for these backhaul applications.
Line-of-sight, LOS, multiple-input multiple-output, MIMO, is a technology which enables communication of several independent data streams over a single frequency band. Thus, the spectral efficiency of a point-to-point radio link using LOS-MIMO technology is potentially much higher than that of a conventional single-input single-output, SISO, system using the same frequency resources, which only communicates a single data stream.
LOS-MIMO communication systems exploit multiple antennas at both ends of a fixed point-to-point radio link in order to achieve said increase in spectral efficiency. However, a prerequisite for enabling this type of MIMO communication is that the antennas of the LOS-MIMO transceiver arrangements are carefully positioned relative to each other, based on the radio link distance and also based on the center frequency of communication. Consequently, LOS-MIMO installations are associated with constraints on relative antenna positions.
From a network operator point of view, it is important that backhaul deployment is straightforward and cost effective. Emphasis is therefore on low cost, few variants, low weight, small footprint, and simplified configuration.
A problem with the LOS-MIMO technique, then, is that the constraints on antenna positions can become prohibitive, and prevent a cost effective and straight forward deployment of the radio link. This issue especially concerns LOS-MIMO communication over large distances and/or at low carrier frequencies, because the necessary antenna spacing on transmit and receive sides of the radio link can become very large; For a 50 km long radio link hop at around 6 GHz center frequency, the ideal antenna spacing is on the order of 35 meters between antennas at both ends of the radio link.